logtail: make logs flush delay dynamic

Instead of a static FlushDelay configuration value, use a FlushDelayFn
function that we invoke every time we decide send logs. This will allow
mobile clients to be more dynamic about when to send logs.

Updates #6768

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f